Wash the black currants well and drain them, then remove the stems.
Pour the black currants, brown rock sugar, and schnapps into a large glass jar, screw the lid on tightly, and place in a sunny window. Let it ripen for 8 weeks, shaking it vigorously 2-3 times per week to help the sugar dissolve.
Line a large sieve with linen cloth and strain the liqueur through it into bottles. Let rest for 1 week before serving.